Replace background auto-refresh with fetch-on-launch + explicit button

Periodic hourly WorkManager fetches and the 5am exact-alarm morning fetch
both refreshed without any visible trigger, which surprised the user more
than it helped. Refreshing now only ever happens on a fresh app launch
(MainActivity.onCreate) or an explicit tap of the new refresh button on the
Unread/Older screens' top bar (wired to the fetch-status state that was
already being computed there but never rendered). Drops MorningFetchReceiver
entirely along with its exact-alarm permission and boot receiver.

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